Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air and/or blow-by gases may be received at a compressor inlet location. Hydrocarbons ingested from purge air, the positive crankcase ventilation (PCV), and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, the accuracy of the sensor may be reduced, and measurement and/or control of EGR may be degraded.
In one example, the issues described above may be addressed by a method for an engine comprising: during operating with EGR flowing, adjusting an EGR valve based on an output of an intake oxygen sensor and PCV flow, the PCV flow identified during previous engine operation with EGR and purge disabled based on outputs of the intake oxygen sensor with and without boost. In this way, the hydrocarbon effect on the sensor from PCV flow can be nullified and the accuracy of EGR estimation can be improved.
For example, during engine operation when EGR is disabled (no EGR is flowing) and purge is disabled (e.g., a fuel canister purge valve is closed), a correction factor for the intake oxygen sensor, based on PCV flow, may be learned. Specifically, the correction factor may be based on a change in intake oxygen concentration (or reading) at the intake oxygen sensor between non-boosted and boosted engine operation. This is because when operating the engine without boost, and with purge disabled, PCV flow is received in the intake manifold directly, downstream of the intake oxygen sensor. Since the sensor output is not affected by the PCV hydrocarbons, the sensor output is reflective of the intake oxygen concentration. In comparison, when operating with boost, and purge disabled, PCV flow is received in the air induction system, upstream of the intake oxygen sensor. Here the sensor output is affected by the PCV hydrocarbons, and the sensor output is reflective of the PCV flow. Therefore, by comparing the sensor outputs with and without boost, a change in intake oxygen resulting from PCV flow may be learned and used to correct subsequent outputs from the sensor. For example, during subsequent engine operation when EGR is flowing and the fuel canister purge valve is closed, an output of the intake manifold oxygen sensor may be adjusted based on the correction factor. As a result, a contribution of PCV flow to intake oxygen may be removed from the output of the sensor. The resulting adjusted output may more accurately reflect the change in intake oxygen resulting from EGR. An EGR flow rate may be estimated based on the adjusted output, and an EGR valve may be accordingly adjusted for improved EGR control. Estimating EGR flow in this way may increase the accuracy of EGR flow rate estimates, thereby increasing EGR system control and maintaining engine emissions at target levels.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.